Nutrient Management Research Database

http://ucanr.edu/sites/Nutrient_Management_Solutions/files/212705.pdf

Almond production is dependent on nitrogen availability. Cover crops offer a potentially valuable source of nitrogen, especially if the cover crop plants are nitrogen fixers, such as clovers, vetches, and other legumes. Little is known about how much N derived from a cover crop is actually taken up by the almond trees, and over what period of time.

We grew six young 'Non-pareil' almond trees in lysimeters with intact columns of orchard soil for eight weeks. To the soil surface of three lysimeters we applied 180 g (DW) of 'Lana' woollypod vetch hay grown with a 15N labeled fertilizer (KN03). Leaf samples were collected bi-weekly and analyzed for %N and 15N content. After eight weeks all of the trees were destructively sampled and the soil, roots, stems, and leaves were analyzed for %N and 15N.

Almond leaf nitrogen derived from a vetch cover crop (%Ndfcc) increased rapidly over four weeks, stabilized at about 30% in existing leaves, and reached 43% in new leaves. Almond leaf nitrogen content increased from 1.6-2.3% over the first four weeks. Leaf biomass doubled in eight weeks compared with control plants grown without vetch hay. Leaf number increased 88% in treated trees and decreased 5% in controls. Soil nitrogen derived from vetch was >4% in the top 4 inches, although absolute soil nitrogen levels decreased in eight weeks. We could account for approximately 75% of nitrogen released from the vetch, about equally divided between soil and plant pools.

These results indicate that substantial nitrogen was released from the vetch residue applied to the soil surface in the treatment lysimeters. Thirty-five percent of the released nitrogen was taken up by the tree in eight weeks following application of the vetch residue. The first four weeks showed the fastest rate of uptake. Vetch nitrogen was partitioned into plant and soil pools approximately equally. On a per biomass basis, and as a total, new leaves represented the greatest uptake of vetch-derived nitrogen. Vetch as a source of nitrogen provided adequate nutrition for young almond trees.

This report summarizes two related experiments on almond uptake of nitrogen from a vetch cover crop.  The first (1995-1997) focused on release of total N from the vetch cover crop.  In the second (1998-1999), Non-pareil almond trees were grown in lysimeters.  Lana woolypod vetch grown with a 15N labeled fertilizer was applied to the soil surface and leaf samples were collected and assessed for %N and 15N concentrations.  At the end of the study, trees were destructively sampled and analyzed for %N and 15N. Consisted irrigation was applied.  

The abstract included here summarized just the 1998-1999 research.  

The experiments conducted in 1997-1998 found that up to 50-75% of N from the vetch cover crop was released in the first 2 weeks of application and up to 90% was released after 4 weeks.  Decomposition rate was directly related to irrigation and moisture.  Loss of N through volatilization represented only about 1% of N released from the cover crop.

The 1998-1999 experiments were conducted with consistent irrigation.  There was a steady increase in the % of leaf nitrogen derived from vetch, increasing to 31% or total nitrogen by week 4 of the study for treatments receiving a vetch cover crop